Method of and apparatus for automatically proportioning granulated material to be fed to a grinding mill



Oct. 23, 1956 L, TRIEF AL 2,767,926

METHOD OF AND APPARATUS R AUTOMATICALLY PROFORTIONING GRANULATED MATERIAL TO BE FED TO A GRINDING MILL Filed Dec. 13. 1949 r 2,767,926 METHOD OF AND APPARATUS FOR AUTOMATI- CALLY PROPORTIONING GRANULATED MA- TERIAL TO BE FED TO A GRINDING lVIILL Leon Trief and Marcel Trief, Brussels, Belgium Application December 13, 1949, Serial No. 132,642 Claims priority, application Belgium December 15, 1948 6 Claims. 01. 241-15 The present invention relates to a method for grinding granulated slag to produce a hydraulic binder, as well as apparatus for carrying out and employing the aforementioned method. It is known to grind blast-furnace slag, after it has been previously dried, in order to obtain a Commercial hydraulic binder or slag cement in powder form, to which a catalyser, generally Portland cement, is added in clinker or powder form, depending upon whether the addition is made during or after the grinding of the slag.

It is also known that the grinding of the granulated slag can be performed by the wet method, so as to obtain, in the form of a paste, a hydraulic binder having high latent hydraulic-binding properties, said properties appearing only after the binder has been put in the presenceof a catalyser such as Portland cement. The aforementioned wet grinding method has been shown to be advantageous over the dry grinding method in that it results in a substantially greater fineness of grinding; a considerable saving in motive power; the possibility of employing slags, the chemical composition and physical structure of which do not enable them to be ground to powder, for example slags of insuificient basicity or slags that are impossible or too costly to dry on acount of their excessive water content or too small specific gravity.

The fineness of grinding is the determining factor of the speed of the hydration of the constituents of a hydraulic binder, and it has been found that very finely ground slag possesses higher qualities than Portland cement, both from the point of view of its mechanical strength and its resistance to attacking agents. Therefore, in order to prepare a hydraulic bindervfrom granulated slag, it is necessary to secure a very fine grinding; however, since the slags which may be employed may show considerable variations in specific gravity, in specific grinding hardness and in water content, it will be understood that, in order to obtain products of uniform quality, it is also necessary to secure the regularity of the fineness of grind- In fact, the specific gravity may vary from 0.5 to 1.5 approximately, viz: from 0.5 to 1 approximately for white to brown slags and from 1 to 1.5 approximately for brown to blue-blackish slags.

With respect to the specific hardness to grinding, this also varies in comparable proportions, the blue-blackish slags being considerably harder than the white slags.

I Further the water content, also varies to a considerable degree, approximately from to 25% or even more, depending upon whether blue-blackish or white slags are concerned.

Even with slag of the same origin, those features are widely divergent, since they depend, inter alia, on the casting temperature of the blast-furnace and on the conditions of carrying out the granulation by projection into a flow of cold water.

The fact that the known grinding plants did not effectuate an even grinding to a high fineness by using slags of variable characteristics will become clear if it is kept in mind that such plants supply the grinding mill with volumetric quantitiesv of the material to be. ground, for example, by means of a measuring hearth or aheli-cal ice 2- worm. Such mills are continuously operating devices, so that, for an equal volume, it was the heaviest slags that were undergoing the less fine grinding. This would have been the case even if all the slags had the same specfic hardness, i. e., resistance to grinding. Now, since the heaviest slags are also the hardest ones, the result is av still greater unevenness in fineness of grinding in the known plants.

Homogenization of the slags prior to their grinding, in view of equalizing the specific gravity, has been employed in prior art devices. However, this resulted in'the serious drawback that materials which were heterogeneous from the standpoint of hardness were ground simultaneously and within the same lapse of time, so that the hardest portions were ground too coarse, which is very detrimental for their properties when used as constituents of a hydraulic binder.

This invention makes it possible to obviate the aforesaid drawbacks, owing to the fact that granulated slag is fed to the mill, without previous homogenization, by quantities of weight that vary in inverse proportion to the hardness of the material to be ground. This avoids simultaneous grinding of heterogeneous materials from the standpoint of hardness and ensures a perfect regu larity of the fineness of grinding, since, the greater the hardness of the materials, the more energetic the grinding action to which they are subjected.

The invention provides a device for automatically carrying out such proportioning of weight, said device comprising means for feeding the mill with a constant weight of slag per unit of time, and means for supplying the mill with an additional weight of slag, variable as inverse function of its hardness, so that the total weight of the slag supplied per unit of time varies in inverse proportion to the hardness. It will be understood that such proportion must not be necessarily observed in a rigorously exact manner; however, one should remain within rather narrow limits, such as are admissible in practice.

The law of variation of the hardness as a function of the specific gravity of the slag being known, all that is necessary is that the additional supply means be driven under the action of components adjusting the volumetric feed of the slag for the constant-weight feed means, so that, for every position of such components, there will be a corresponding suitable delivery from the said additional supply means.

This method of feeding may be applied to both the dry and the wet grinding of slag. It may also be applied for a similar treatment of materials the feeds of which need be varied in gravity, as a function not only of the specific weight, but also of another factor which varies with the specific gravity.

In the case of wet grinding, the evenness of grinding, the obtention of which is the essential aim of the invention for the above mentioned reasons, depends not only on the supply of solid materials but also on the consistency of the formed paste, for example, on its water con tent, which influences the speed of travel across the grinding mill.

If the percentage of water is too small, the mill will be blocked up, while, if the percentage is too high, the material will be discharged too quickly. In both cases, the regularity of the fineness of grinding is unfavourably influenced.

The Water content of the slag Varies largely according to its physical condition and according to whether or not it has been exposed to the inclemencies of the weather. Therefore, in order to obtain a paste of regular consistency, the quantity of water added to the mill should vary accordingly. It is known to adjust this quantity of water intermittently by hand and in accordance with the ascertained variation of the fluidity of the paste. This way of Pa e e O 3 419 proceeding requires a constant supervision and is not satisfactory because corrections are only made in an approximate manner and with substantial delay.

The invention obviates the aforesaid serious drawback by providing for the automatic control of the additional water, so as to keep the total percentage of water in the paste practically constant.

To this end the invention provides that the quantity of additional water will be controlled as a function of the variations of the quantity of motive power absorbed by the mill. The current absorbed by an eiectric motor driving the mill may for instance influence a solenoid, the armature of which operates, through a suitable transmission, the displacements of a valve controlling the additional quantity of water. All other conditions remaining equal, the said current, will vary as a function of the consistency of the paste, i. e. as a function of its water content, and it will control automatically the variations of the quantity of additional water as a function of its own variations.

It will be understood that, in order to achieve perfect results, by so controlling the water, it is necessary that the supply of slag be controlled as a function of its specific weight and its hardness, so that the energy necessary for its grinding proper remains constant. Such condition is fulfilled by the first object of the invention, which has been described above, so that the combination of the two control means, respectively of the slag and of the water is practically necessary in order to achieve a wet grinding with a regular fineness.

The combination of the aforementioned controls is also advantageous from the point of view of the thermal conditions of the grinding.

The wet grinding of the slag considerably reduces the friction and, therefore, the increase of temperature in the mill, the water acting as a lubricant and absorbing moreover a large proportion of the generated calories. However, when a fineness of grinding is achieved, which is of the order of those obtained according to the invention, and which often exceed a specific surface of 3000 cmF/gn, the temperature in the mill may increase so as to be 30 to 40 C. in excess of the surrounding temperature.

Especially in summer time and in warm countries, the aforesaid increase of temperature may cause deterioration f the paste, and even concreting of the mill. The cooling of the mill is therefore an imperative necessity, and has been carried out more or less successfully by known means, such as applying ribs on the body of the mill, providing a cooling sleeve at the discharge point of the paste, refrigerating the feed water and injecting cold air. However, whatever he the case, such means do not compensate the variable excessive increases of temperature due to the irregular feed to the mill, particularly as far as regards the hardness of the material to be ground and, more specifically the variations of the water content of the paste.

The combined application of the af rementioned controls, by which the energy spent for grinding is smoothed out, eliminates the variable excessive increases of temperature in the mill. In order to reduce the so obtained regular increase of temperature, the invention provides the use, simultaneously with one or more of the known but insufficient means, of an exhaustion, preferably at the point of discharge of the paste, of the produced hot air and vapour. Such exhaustion of the vapour is important not only because it removes a large number of calories, but mainly because it prevents any condensation of the vapour, especially when the mill is inoperative. Such condensation may considerably increase the water content of the paste, namely to such a point that, when the mill is again set into motion, the excessive water content may result in a highly insufficient fineness of grinding. It will be clear, therefore, that the use of the additional cooling means in accordance with the invention assists in equalizing the fineness of grinding by the wet method. Its use combined with the first two objects of the invention prevents all impairing with the effect of equalization produced by the first two controls and forms, together with the said first two controls, a coherent sys tem, capable of securing a perfect regularity of the fineness of grinding.

By way of illustration one example of carrying out the invention is hereinafter described as applied to a plant for wet grinding granulated slag, with reference to the accompanying diagrammatic drawings, wherein:

Fig. 1 is a schematic view of a wet grinding installation according to the present invention and including a mill and a proportioning device for granulated slag, and

Fig. 2 is a fragmentary view on a larger scale of a detail of the proportioning device.

In the drawing, 1 indicates a hopper or tank into which the substance to be ground is introduced and the discharge of which is controlled by a movable closure member 2 actuated by a reversible servo-motor 4, through the intermediary of a rod 3. A vibrator 5 mounted upon a base 6 provided with springs 7, loosens the contents of the hopper in order to facilitate the flow. Under the hopper there is mounted a conveyor band 8 guided on rollers 9, 10 supported by a frame 11 on which there are also mounted the drive means (not shown) for the conveyor band. The frame is mounted like a pivoted beam, insomuch as its one end rests on a stationary knife edge 12, while its other end is suspended on one end of a lever arm of a scale 13, the other end of which supports an adjustable weight 14. On both sides of the lever arm there are provided two spring contacts 15, 16, which are selectively engagable with a contact 27 for completing a energization circuit for the motor 4 to cause rotation thereof in one direction or the other, depending upon which of the contacts 15, 16 engages the contact 27. Any suitable known electrical circuit arrangement can be provided for selectively connecting the energization circuit of the motor 4 to the current terminals 28, 29, one of said circuits being illustrated in Figure 1.

The delivery from the hopper 1 takes place through two discharge means or openings 17 and 18, the first of which projects the slag onto the conveyer band 8, while the other projects the material into a separate discharge device 19 arranged beside and spaced from the conveyer band. The two quantities of slag so delivered are continually conveyed to a grinding mill 22 by any means, such as the hopper 30 which is arranged below and adjacent the ends of the conveyer band 8 and the discharge device 19 and communicates with the grinding mill. The fraction delivered from the opening 17 and conveyed by the band 8 is maintained automatically at a constant weight (adjustable by means of the weight 14), whatever be the variations in specific gravity of the granulated slag issuing from the hopper 1.

In fact, if the specific gravity increases or decreases, the lever arm of the scale 13 acts upon the contact 15 or 16 so as to displace the closure member 2 in the direction of closing or opening, through the medium of the servomotor 4, until the closure member will occupy the position securing the delivery of the requisite constant weight.

The shape and the size of the opening 18 are determined as a function of those of the opening 17, so that the total quantity discharged through the two openings varies, for the various positions of the closure member, at least substantially in inverse proportion of the hardness of the slag. If the hardness decreases with the specific gravity, the opening 18 will increase simultaneously with opening 17. The shape of the opening 18 will depend on the law that interlinks the specific gravity and the hardness of the slag. The shape of opening 18 may be substantially triangular and is made to suit the particular range of the products to be treated. Instead of being formed in the same plate as the opening 17, the opening 18 may be formed in a separate plate which may be made to fit into a slide formed in the bottom of the hopper 1, thereby making it easy to vary the opening 18.

The wet grinding mill to which the two aforesaid quantities of slag are fed should also be provided with control means for the additional water admitted to the mill, so as to maintain an equal consistency of the paste, which is a necessary condition to obtain a good regularity-of the finess of grmdlng. The said control is effected as a function of the power spent for operating the mill, the slagfeed to which is controlled as above described. In Figure 1 there is shown a simple arrangement which consists in mounting a solenoid 20 in the main circuit of the electric drive motor 21'of the mill 22. The movable armature of the solenoid 23 controls through the medium of a set of rods 24, a valve 25 that controls the delivery of the additional water, so as to increase or reduce such delivery according to the sense of variation of current in the circuit of the drive motor of the mill." The hot air and vapour produced in the mill may be withdrawn through an exhaust pipe 26.

It will be understood that the invention is not limited to the above described examples of execution, but that any other appropriate means may be used in order to obtain the results'sought and above specified.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent, is:

1. A device for automatically proportioning granulated material of varying specific gravity where the hardness of said material varies in a known relation with respect to said specific gravity, comprising constant-weight scale means including a pivotally mounted frame and a scale arm operatively connected with said frame, a conveyor carried by said frame and arranged for delivery of said material when deposited thereon to a receiving station, a hopper for said material, a discharge device located adjacent said frame and arranged for delivery of additional material when received thereby to said receiving station, said hopper being provided with first and second discharge openings spaced from each other, said first opening being located above said conveyor and said second opening being located above said discharge device, a closure member movable past both said openings to close respective portions of the latter, an actuating member for said closure member, and means responsive to movement of said frame and said scale arm for operating said actuating member so as to ensure delivery to said conveyor during a selected unit of time of a constant weight of said material regardless of movement of said closure member relative to said first opening in response to variation of said specific gravity, the shape of said second opening being chosen to establish a predetermined areal relationship between said openings, whereby in each position of said closure member relative to said openings in accordance with said variation of specific gravity of said material passing through said first opening, the quantity of said material passing through said second opening during said selected unit of time is of suflicient weight to ensure that the total weight of said material delivered by said conveyor and said discharge device to said receiving station during said selected unit of time varies substantially inversely with said hardness of said material.

2. A device for automatically proportioning granulated material of varying specific gravity where the hardness of said material varies in a known relation with respect to said specific gravity, comprising a hopper for said material, a first conveyor arranged for delivery of a first quantity of said material when deposited thereon to a receiving station, a second conveyor located adjacent said first conveyor and arranged for delivery of a second quantity of said material when received thereby to said receiving station, said hopper being provided with spaced first and second discharge openings, said first Opening being located above said first conveyor and said second opening being located above said second conveyor, movable closure means for variably obturating both said openings, and constant-weight scale means operatively connected to both said conveyor and said closure means for moving the latter with respect to said openings in response to variations in specific gravity of said first quantity of said material passing through said first opening, to thereby en- "sure tleliveryfto said first conveyor of a'con stant Weight 'of said material perunitof time irrespective of the movement of said closure member relative to said first opening due'to said variation in specific gravity of said first quantityof said material, therelationship between the widths of said openings being so predetermined that, in each position of said closure member relative thereto in accordance with said variation of specific gravity of said material passing through saidfirst opening, said second quantity of said material passing through said second opening is of sufiicient weight to ensure that the total weight of said material delivered by said first and second conveyors to said receiving station per unit of time varies substantially inversely with said hardness of said material.

3. In a method for automatically proportioning a granular material to be fed to a grinding mill, said material having a varying specific gravity and a specific hardness varying in a known relation with respect to said specific gravity; the steps of feeding a substantially constant weight of said material per unit of time as a first material delivery to one station, simultaneously supplying to a second station an additional weight of said material as a second delivery, while automatically varying the weight of said second delivery in response to the specific gravity of said material and in a predetermined relation thereto, said last-named relation being determined in dependence of said relation between the specific gravity and the hardness of said material, whereby the total weight per unit of time for both deliveries varies substantially in inverse proportion to the hardness of the material, and finally transferring both deliveries from their stations to said grinding mill.

4. In a method for preparing in a power-driven grinding mill a substantially uniform particle fineness from a granular material of varying specific gravity and having a specific hardness varying in a known relation with respect to said specific gravity of said material; the steps of feeding a substantially constant weight of said material per unit of time as a first material delivery to one station, simultaneously supplying to a second station an additional weight of said material as a second delivery, while automatically varying the weight of said second delivery in response to said specific gravity of said material and in a predetermined relation thereto, said last-named relation being determined in dependence of said relation between the specific gravity and the hardness of said material, whereby the total Weight per unit of time for both deliveries varies substantially in inverse proportion to the hardness of the material, simultaneously transferring both deliveries from their stations to said power-driven grinding mill, introducing a liquid in continuous flow to the material to be ground in the grinding mill, and controlling the introduction of said liquid as a direct function of the motive power consumed by said mill, whereby said motive power may be maintained substantially constant.

5. A device for the automatic proportioning of granulated material of varying specific gravity, the specific hardness of said material varying in a known relation with respect to said specific gravity, comprising a source for supplying said material, first discharge means for delivering material from said source, means responsive to variations in said specific gravity of said material for controlling said first discharge means so as to enable the latter to deliver a substantially constant weight of material per unit of time, second discharge means for simultaneously delivering from said source a variable additional weight of said material, means for automatically controlling said second discharge means as to the quantity of said material delivered by said second discharge means in response to said variations in specific gravity of said material delivered by said first discharge means and in such predetermined relationship to the quantity of said material delivered by said first discharge means, that the total weight of both deliveries from said first and said second discharge means per unit of time varies substantially in inverse proportion to the hardness of the material.

6. A device for the automatic proportioning of grannlated material of varying specific gravity, the specific hardness of said material varying in a known relation with respect to said specific gravity, comprising an adjustable constant-weight scale having a pivotal arm, a continuous conveyor suspended on said arm, a tank for the granulated material, said tank being provided with a substantially rectangular first outlet-opening located above said continuous conveyor, a second conveyor, said tank being further provided with a gradually widening second outlet-opening located above said second conveyor, a movable closure member for controlling the respective free areas of said openings, an actuating member connected to said closure member, means responsive to movements of said arm for operating said actuating member, the shapes of said two openings being interrelated in each of the various positions of said closure member according to said relation between the specific gravity and the hardness of said material, whereby the weight of the total delivery from the two openings varies substantially in inverse proportion to the hardness of the material.

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